1. Field of the Invention
The present invention relates to an origin location detection circuit in an encoder. The encoder includes a scale on which an origin pattern indicating an origin is provided, and a detection head relatively displaceable with respect to the scale. The origin location detection circuit includes a comparator for generating an origin signal indicating a location of the origin from an output voltage signal, which is output from a detector of the detection head and has been affected by involvement of the origin pattern. In particular, the present invention relates to an origin location detection circuit capable of improving reproducibility of a detection location of the origin signal even when there is fluctuation in a DC component of the output voltage signal.
2. Description of Related Art
Conventionally, a detection method of an origin location in a photoelectric encoder, such as the one disclosed in Japanese Patent Laid-Open Publication No. 2000-97726, has been proposed. In such a detection method of the origin location, for example, when an LED as a light source and a phototransistor as a light receiver (detector) are used, an origin location detection circuit as illustrated in FIG. 10(A) is a basic configuration. In FIG. 10(A), a resistor R2, which acts as a load, is connected between an emitter end (output end) of the phototransistor, which is a light receiver 26, and the ground. The output end of the phototransistor is connected to a minus input terminal of a comparator 32, and a threshold voltage Zsh, obtained by dividing a power-supply voltage VDD using resistors R10 and R11, is input to a plus input terminal of the comparator 32. The threshold voltage Zsh does not change. Therefore, in this origin location detection circuit, an output voltage signal φZ (which has been affected by the involvement of the origin pattern) output from the light receiver 26 is compared with the threshold voltage Zsh, which is of a constant value, at the comparator 32, and is converted to a digital signal, and thereby an origin signal PZ indicating an origin location is generated. The origin signal PZ from the origin location detection circuit is used, during power-on, to recognize an absolute location within a movement range of a control machine (movement stage) on which the detection head is mounted.
However, as illustrated by a dashed line in FIG. 10(B), a DC component of the output voltage signal φZ fluctuates (ΔφZ) under the influence of degradation of the LED and phototransistor, fluctuation in ambient temperature, fluctuation in the power-supply voltage VDD, location changes over time of the scale and detection head of the photoelectric encoder (fluctuation in a control machine or in a bonding state between the detection head and the control machine), and the like (the DC component of the output voltage signal φZ means an approximately DC voltage component containing a moderate voltage fluctuation excluding fluctuation in the output voltage signal φZ at the origin location (this includes fluctuations in other transient output voltage signals φZ of relatively fast periods)). Therefore, the origin signal PZ changes, and, as a result, a detection location deviation DV of the origin signal occurs. Such a circumstance has been newly discovered by the inventors (a symbol TSW in FIG. 10(B) indicates a two-phase sinusoidal signal generated by an incremental pattern (INC pattern) of the scale).
It is also conceivable to follow the fluctuation in the DC component by using a high pass filter disclosed in Japanese Patent Laid-Open Publication No. 2007-240348. In this case, a frequency component obtained when detecting an origin signal using the high pass filter of Japanese Patent Laid-Open Publication No. 2007-240348 must be separated from a frequency component related to the DC component. However, in a case where a location of an origin is actually completely unknown during power-on, a detection operation is extremely slowly performed when detecting the origin location of an encoder, which is not limited to a photoelectric type. In such a case, a frequency component obtained when detecting an origin signal contains a significantly low frequency component. Therefore, it is difficult to separate the frequency component obtained when detecting an origin signal using the above described high pass filter and the frequency component related to the DC component. That is, using the technology of Japanese Patent Laid-Open Publication No. 2007-240348, it is difficult to surely avoid the detection location deviation of the origin signal.